This proposal is for a competitive revision, under NCRR funding announcement: PAR-12-046, of the parent grant R41RR016292 funding, P41 Center: National Biomedical Center for Advanced ESR Technology. This proposal is for a new technology project, not included on the parent grant, but which greatly benefits the aims of the parent grant, in particular Technology Research and Development (TR&D) Project: Structure of Proteins and Protein Complexes by Pulse Dipolar ESR (PDS-ESR), which is concerned with improving existing and developing new distance measurement techniques for structural studies on biomolecules. The new project is to develop methods of composite pulses, common in NMR, to be applied to ESR, which requires much shorter pulse widths and is subject to much faster spin-relaxation. This is now possible using a new Tektronix Arbitrary Waveform Generator (AWG) that will enable the modulation of the microwave pulses with sub-ns resolution. It will enable better spectral coverage by the pulses in the double-quantum coherence (DQC)-ESR method for distance measurements, which requires non-selective pulses. It will enable well-designed selective pulses required for the double electron-electron resonance (DEER) method. This will enhance signal-to-noise and suppression of unwanted signals and artifacts, thereby improving these methodologies. The AWG will be integrated into the pulse ESR spectrometer with due consideration of the letter's detailed performance. Monitoring of the composite pulsing at key positions in the microwave bridge, with the help of a sufficiently wide-bandwidth digital sampling oscilloscope (DSO), will permit automatic retuning of the AWG to modify as needed the composite pulses to guarantee the quality of the pulses that actually reach the sample. This project will involve a synergistic interaction, driven by the needs of the Driving Biomedical Projects (DBP's) as in the Parent Grant. Careful tests will be made of this new composite-pulse technology to demonstrate how it significantly improves the distance measurements and allows new and more challenging studies included in the DBP's to be carried out.